1. Field of the Invention
The present invention is directed toward the field of television tuning, and more particularly toward a baseband filter for demodulating a television signal.
2. Art Background
In general, televisions include circuits to demodulate radio frequency television signals to generate video and sound signals. The video and sound signals provide the information necessary to form the television picture and sound, respectively. An ultrahigh frequency (“UHF”)/very high frequency (“VHF”) tuner is one type of circuit found in television receivers. In general, the UHF/VHF tuner receives a radio frequency (“RF”) television signal that includes a plurality of channels. The channels are modulated on a carrier frequency. The carrier frequency may be in the UHF spectrum or the VHF spectrum. The television is set or tuned to receive a specific channel (e.g., channel 2). The U/V tuner processes the RF television signal based on the channel selected, and generates an intermediate frequency (“IF”) signal. In the United States, the intermediate frequency, used in television receivers, is set to a frequency of 45.75 Mhz.
Television receivers also include circuits to perform intermediate frequency processing. These IF television circuits typically employ surface acoustic wave (“SAW”) filters. The SAW filter conditions the IF signal prior to demodulation (i.e., prior to extracting the video and audio signals). The SAW filter rejects or suppresses the energy bands associated with channels adjacent to the desired channel (i.e., the selected channel). To this end, the SAW filter provides a Nyquist slope bandpass response for the IF signal.
FIG. 1 is a block diagram illustrating one embodiment for a prior art television receiver. As shown in FIG. 1, the U/V tuner 110 conditions and converts the RF signal at the tuning frequency to the intermediate frequency (IF) signal. The IF signal is input to the SAW filter 120. The output signal from SAW filter 120 is input to an IF processor 130. In general, IF processor 130 demodulates the television signal to generate baseband video and audio signals.
As discussed above, the SAW filter provides a Nyquist slope response. FIG. 2a illustrates various Nyquist slope responses. As shown in FIG. 2a, slope 200 depicts the ideal Nyquist slope. Note that the ideal Nyquist slope crosses at the picture frequency (Fp) at 0.5 of the maximum energy of the filter response. FIG. 2a also shows two non-ideal Nyquist slopes. As shown in FIG. 2a, the response of slope 210 crosses the picture frequency (Fp) at a lower point than the ideal Nyquist slope (i.e., slope 200). Conversely, slope 220 crosses the picture frequency (Fp) at a point higher than the ideal Nyquist slope.
FIG. 2b illustrates various waveform responses as a result of the SAW filter. The ideal waveform response, waveform 230, is a result of the SAW filter providing an ideal Nyquist slope (i.e., slope 200 of FIG. 2a). The waveform response 240, which includes additional out of band energy, is a result of the non-ideal Nyquist slope 210 shown in FIG. 2a. Also, waveform response 250, which filters the signal in the information band, is a result of the non-ideal Nyquist slope 220 of FIG. 2a. 
When using a SAW filter in the television receiver, the non-ideal Nyquist slopes (210 and 220, FIG. 2a) and corresponding waveform responses (240 and 250, FIG. 2b) are a result of off tuning. Specifically, if the SAW filter is not tuned to filter at the appropriate center frequency, shifts in the Nyquist slope (e.g., Nyquist slopes 210 and 220) occur. In turn, this off tuning of the SAW filter provides the undesirable waveform responses (e.g., waveforms 240 and 250, FIG. 2b).
Also, as shown in FIG. 1, the television circuit includes the automatic frequency tracking detection circuit 140. In general, the automatic frequency tracking (AFT) detection circuit 140 determines, based on the baseband audio and video signals, an offset between the actual carrier frequency of the tuned signal and the frequency of the local oscillators in the television receiver. For example, the television receiver circuit may process a signal input with a carrier frequency of 90 MHz. For this example, the AFT detection circuit 140 may generate an offset of 0.2 Mhz (i.e., the actual carrier frequency is 0.2 Mhz different than the frequency of the local oscillator in the television receiver.) Based on the feedback, the UV tuner circuit 110 compensates for the offset to more accurately track the carrier frequency. In addition, as shown in FIG. 1, the AFT detection circuit 140 provides tracking information to SAW filter 120. Specifically, the tracking information tunes the SAW filter 120 to provide a frequency response centered around the tracked IF frequency.
It is advantageous to generate a Nyquist slope response in a filter that eliminates the undesirable characteristics introduced through use of a SAW filter.